A, B or O: What's the Difference?

The ABO system has two features unique among blood-group systems: the antigens (A, B and H), when present, are widely distributed in the cells and fluids of the body; and the antibodies (anti-A, anti-B and anti-H) occur regularly in the plasma of people who lack the corresponding antigens. ABO incompatibility is thus a major immunologic hazard, capable of causing hemolytic transfusion reactions, hemolytic disease of the newborn and destruction of grafted tissues. Surprisingly enough, the structural difference between the A and B antigens is very small. Group A specificity is determined by α-linked N-acetyl-D-galactosamine, and B specificity by a-linked D-galactose. . . .